Rydberg atoms have the electrons in their outer layers excited until the electrons are only weakly bound to the nucleus, making the atoms physically very large. The increased size allows light to scatter off the outermost electrons without much interference from the nucleus or from the inner core of electrons. In other words, it’s a way to isolate the electron dynamics from other messy phenomena. Noble gases like argon are particularly useful for this, since they are largely non-reactive chemically and relatively easy to model theoretically. [Read more….]